The present invention relates to an organic electroluminescent device (hereinafter referred to as xe2x80x9can organic EL devicexe2x80x9d). More precisely, it relates to an organic EL device suitable to light sources for displays, printer heads and others for domestic and industrial use.
Some conventional organic EL devices are disclosed in Reference 1 xe2x80x9cJapanese Patent Laid-Open No. 312873/1989xe2x80x9d, Reference 2 xe2x80x9cJapanese Patent Laid-Open No. 207488/1990xe2x80x9d, Reference 3 xe2x80x9cJapanese Patent Laid-Open No. 41285/1993xe2x80x9d, and Reference 4 xe2x80x9cJapanese Patent Laid-Open No. 119973/1994xe2x80x9d. The organic EL devices disclosed in those references have a laminate structure comprising an inorganic semiconductor layer that serves as a hole injection layer or an electron injection layer, and an organic light-emitting layer. In those, the inorganic semiconductor layer is degraded less than the organic layer. With the structure of that type, the organic EL devices could have a prolonged life.
In Reference 1, amorphous materials of III-V Group or II-V Group, for example, represented by Si1-xCx, or crystalline materials such as CuI, CuS, As, ZnTe and the like are used for the inorganic semiconductor layer.
In Reference 3 and Reference 4, disclosed are examples of using crystalline oxide semiconductor materials such as typically Cu2O for the inorganic semiconductor layer.
However, in the organic EL devices disclosed in Reference 1 and Reference 2, the inorganic semiconductor layer of a crystalline material such as CuI or the like generally has a polycrystalline morphology. In those, the surface of the polycrystalline, inorganic semiconductor layer has poor planarity, having a surface roughness of the order of around 50 nm or more. In those, therefore, when a thin film of an organic light-emitting layer is formed on the polycrystalline, inorganic semiconductor layer, the projections of the rough surface of the inorganic semiconductor layer would often penetrate through the thin film. In that case, the inorganic semiconductor layer will short-circuit with the electrode on the organic light-emitting layer to give a leak current. Even if they do not short-circuit with each other, the projections of the rough surface of the inorganic semiconductor layer will yield the concentration of electric field around them, thereby also giving a leak current. For these reasons, the conventional organic EL devices have the problem of the low luminous efficiency depression.
In forming the inorganic semiconductor layer, it is heated at a temperature higher than the temperature at which the organic light-emitting layer is stable and safe. Therefore, the light-emitting layer shall be formed after the inorganic semiconductor layer has been formed.
In addition, in the organic EL devices disclosed in Reference 1 and Reference 2, the energy gap of the amorphous materials of Si1-xCx used is smaller than 2.6 eV. On the contrary, the energy gap of the organic light-emitting layer that contains a light-emitting material of aluminium complexes or stilbene derivatives is larger than 2.6 eV. As a result, the excited state having been produced in the organic light-emitting layer is often quenched through energy transfer from the organic light-emitting layer to the inorganic semiconductor layer. Accordingly, the organic EL devices have the problem of the low luminous efficiency.
What is more, where amorphous silicon materials (e.g., xcex1-Si, xcex1-SiC) are used for the inorganic semiconductor layer in those organic EL devices, the local level attributed to the dangling bonds in the energy band gap will be on the order of at least 1017 cmxe2x88x923. In that condition, even if the band gap energy is large, the excited state will be quenched owing to the local level. Accordingly, the organic EL devices have the problem of the low luminous efficiency.
On the other hand, oxide conductors such as Cu2O and others used in Reference 3 and Reference 4 are crystalline substances. In those, the oxide conductors such as Cu2O and others are baked at high temperatures and, therefore, generally have a polycrystalline morphology. Accordingly, for the same reasons as in the case of Reference 1 and Reference 2 noted above, the organic EL devices in Reference 3 and Reference 4 also have the problem of the low luminous efficiency in that the rough surface of the polycrystalline, inorganic semiconductor layer in those organic EL devices produces a leak current.
Taking account of the problems noted above, we, the present inventors have achieved the invention. The object of the invention is to provide an organic EL device with high luminous efficiency.
To attain the object, the organic EL device of the invention has a structure of a first electrode layer, an inorganic non-degenerate semiconductor layer, at least one organic layer including a light-emitting layer, and a second electrode layer as laminated in that order, and is characterized in that;
the inorganic non-degenerate semiconductor layer includes an amorphous material or a microcrystalline material, and its band gap energy is higher than that of the organic light-emitting layer.
In the organic EL device of the invention, the inorganic non-degenerate semiconductor layer includes an amorphous material or a microcrystalline material. Accordingly, in this, the surface of the inorganic non-degenerate semiconductor layer is planarized. As a result, in this, the inorganic non-degenerate semiconductor layer with no surface roughness is prevented from producing a leak current. Accordingly, the luminous efficiency of the organic EL device with that structure is improved.
In the organic EL device of the invention, the band gap energy of the inorganic non-degenerate semiconductor layer is higher than that of the organic light-emitting layer. As a result, in the organic EL device, the excited state having been produced in the organic light-emitting layer is prevented from being quenched through energy transfer from the organic light-emitting layer to the inorganic non-degenerate semiconductor layer. Accordingly, the luminous efficiency of the organic EL device with that structure is improved.
In the organic EL device of the invention, the band gap energy of the inorganic non-degenerate semiconductor layer preferably falls between 2.7 eV and 6 eV.
As so mentioned hereinabove, the energy gap of the organic light-emitting layer including a light-emitting material of aluminium complexes or stilbene derivatives is larger than 2.6 eV. Therefore, in the organic EL device of the invention in which the band gap energy of the inorganic non-degenerate semiconductor layer is at least 2.7 eV, the excited state of the organic light-emitting layer is prevented from being quenched.
Also preferably, in the organic EL device of the invention, the inorganic non-degenerate semiconductor layer is for hole conduction. Specifically, in this, the inorganic non-degenerate semiconductor layer may function as a hole injection layer.
Also preferably, in the organic EL device of the invention, the inorganic non-degenerate semiconductor layer is for electron conduction. Specifically, in this, the inorganic non-degenerate semiconductor layer may function as an electron injection layer.
In carrying out the invention, it is also preferable that the inorganic non-degenerate semiconductor layer in the organic EL device comprises, as the main component, an oxide or oxynitride of at least one element of Ba (barium), Ca (calcium), Sr (strontium), Yb (ytterbium), Al (aluminium), Ga (gallium), In (indium), Li (lithium), Na (sodium), Cd (cadmium), Mg (magnesium), Si (silicon), Ta (tantalum), Sb (antimony) and Zn (zinc).
In carrying out the invention, it is more preferable that the inorganic non-degenerate semiconductor layer in the organic EL device is of an oxide or oxynitride of an element combination of any of a combination of In and Zn, a combination of In, Zn and Al, a combination of Al, Zn and Si, a combination of In, Zn and Yb, a combination of In, Zn and Ta or the like.
In the invention, it is also preferable that the carrier concentration in the inorganic non-degenerate semiconductor layer falls between 1019 cmxe2x88x923 and 1012 cmxe2x88x923.
In the organic EL device in which the carrier concentration in the inorganic non-degenerate semiconductor layer is so reduced as to fall within the defined range, the possibility of interaction between the inorganic semiconductor and the excited states of the organic light-emitting layer is reduced. As a result, the luminous efficiency of the organic EL device is prevented from being lowered.
In the invention, it is still desirable that the local level density in the inorganic non-degenerate semiconductor layer is smaller than 1017 cmxe2x88x923.
In the organic EL device in which the local level density in the inorganic non-degenerate semiconductor layer is smaller than 1017 cmxe2x88x923, the local level of that order does not cause inactivation of the excited state of the organic light-emitting layer.
In the invention, it is further desirable that the inorganic non-degenerate semiconductor layer is of an oxide of essentially In.